This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Japanese Patent Application No. 193570/1999 filed in Japan on Jul. 7, 1999 and Japanese Patent Application No. 299827/1999 filed in Japan on Oct. 21, 1999; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a process for preparation of crystallized glass having high stiffness suitable for information recording disks such as magnetic disk substrates or the like and obtaining high surface smoothness and high surface flatness. The present invention further relates to crystallized glass, obtained by the process for preparation according to the present invention, having excellent surface smoothness and surface flatness, to a substrate for information recording medium composed of the crystallized glass, and to a magnetic disk using the substrate.
2. Description of the Related Art
Major structural components of magnetic storage apparatuses such as computers and the like are a magnetic recording medium and a magnetic head for magnetic recording and reproducing. Flexible disks and hard disks have been known as magnetic recording media. As a material of a substrate for the hard disk, aluminum alloy is mainly utilized. In addition, recently, a flying amount of magnetic heads is markedly reduced along with higher densification of recording of personal computers or hard disk drives for servers (lower flying of the magnetic heads). Accordingly, extremely high precision has been demanded for the surface smoothness of the magnetic disk substrates. In the case of aluminum alloy, however, it is difficult to produce a highly flat surface more than a certain level, even with a polishing process using highly precise abrasives and working apparatuses, because the polished surface suffers from plastic deformation due to the low hardness of the aluminum alloy. As TPI (Track per Inch) and BPI (Bit Per Inch) of the magnetic disk recording are made higher, or revolution speed of the disk is made higher, it has also been required to greatly reduce deflexion or vibration of the substrate for magnetic disks at the time of revolution at a high speed, thus to lower a number (TMR) of reading errors of data due to the vibration or flexion. Since aluminum alloy has low strength and low stiffness, however, it is difficult to keep the TMR of a predetermined low level required by specification of hard disk drive.
Therefore, crystallized glass substrates for magnetic disks having high stiffness have been developed instead of the aforementioned aluminum substrates. For example, U.S. Pat. No. 5,476,821 discloses crystallized glass for disks comprising oxide components, indicated in terms of % by weight, of SiO2: 35-60%, Al2O3: 20-35%, MgO: 0-25%, ZnO: 0-25%, provided that MgO+ZnO is more than 10% (MgO+ZnO greater than 10%) , TiO2: 0-20%, ZrO2: 0-10%, Li2O: 0-2%, and NiO: 0-8%, provided that TiO2+ZrO2+NiO is more than 5% (TiO2+ZrO2+NiO greater than 5%) the crystallized glass containing spinel crystal particles as a main crystal. In addition, U.S. Pat. No. 5,491,116 discloses crystallized glass for disks comprising oxide components, indicated in terms of % by weight, of SiO2: 35-60%, Al2O3: 10-30%, MgO: 12-30%, ZnO: 0-10%, TiO2: 5-20%, NiO: 0-8% and the like, the crystallized glass containing spinel crystal particles or enstatite crystal particles as main crystals.
Although these crystallized glasses have a higher Young""s modulus of approximately 140 GPa, crystal particles thus contained are larger, so there occurs such a defect that controlling a surface roughness to be 0.5 nm or lower in Ra (JIS B0601) is difficult. Even if the Young""s modulus is higher, such as approximately 140 GPa, it is impossible to control the head-flying amount, thereby being incapable of responding to higher recording densification.
It is an object of the present invention to provide crystallized glass maintaining a characteristic of high stiffness ( greater than 130 GPa) suitable as substrates for information recording media such as magnetic recording media or the like, the crystallized glass having excellent properties such as a high surface smoothness, a high flatness and the like, that is, the crystallized glass capable of controlling the surface roughness to be 0.5 nm or lower in Ra (JIS B0601).
In order to achieve the above objects, the present inventors found, as a result of studies based on various experiments, that crystallized glass which is suitable as substrates for information recording media having a high Young""s modulus of 140 GPa or higher and a surface roughness of 0.5 nm or lower in Ra (JIS B0601) can be obtained by subjecting glass containing TiO2 as a crystal nucleating agent to heat treatment at a temperature in a range from a temperature 30xc2x0 C. lower than the glass transition temperature, or Tg, to a temperature 60xc2x0 C. higher than the glass transition temperature, or Tg, thus to be homogeneously phase-separated and then by crystallizing the resulting glass, and consequently, the present invention has been completed.
Furthermore, the present inventors found that it is preferable, for providing the crystallized glass suitable as substrates for information recording media having the higher Young""s modulus of 140 GPa or higher as well as the surface roughness of 0.5 nm or lower in Ra (JIS B0601), to heat up the aforementioned phase-separated glass to a temperature for crystallization (for example, 850xc2x0 C. to 1150xc2x0 C.; more preferably, 900xc2x0 C. to 1100xc2x0 C.) at a heating rate of 10xc2x0 C./min or less.
In other words, the present invention relates to a process for preparation of crystallized glass obtained by making a glass containing TiO2 subjected to a phase separation step and a crystallization step, where the phase separation step has heat treatment of the glass at a temperature in a range from a temperature 30xc2x0 C. lower than the glass transition temperature Tg of the glass (Tgxe2x88x9230xc2x0 C.) to a temperature 60xc2x0 C. higher than the glass transition temperature Tg (Tg+60xc2x0 C.), more preferably from the glass transition temperature Tg to a temperature 60xc2x0 C. higher than the glass transition temperature.
Furthermore, the present invention relates to crystallized glass obtainable by making a starting glass containing TiO2 subjected to a phase separation step and a crystallization step, where the phase separation step has heat treatment of the glass at a temperature in a range from a temperature 30xc2x0 C. lower than the glass transition temperature of the glass to a temperature 60xc2x0 C. higher than the glass transition temperature.
According to the present invention, a plate-shaped crystallized glass, especially, a disk-shaped crystallized glass plate can be obtained.
The present invention further comprises: a substrate for information recording medium, composed of a disk-shaped crystallized glass plate, characterized in that a mean particle size of the crystal particles is in the range of equal to or less than 100 nm; a substrate for information recording medium, composed of a disk-shaped crystallized glass plate, characterized in that a transparency at a wavelength of 600 nm is 40% or higher; and a substrate for information recording medium, composed of a disk-shaped crystallized glass plate, characterized in that a polished surface having a surface roughness Ra (JIS B0601) of 1 nm or lower are provided.
In addition, the present invention relates to a process for preparation of a magnetic disk having a magnetic layer at least as a recording layer formed on a disk-shaped crystallized glass plate, and to a magnetic disk having a magnetic layer at least as a recording layer on the substrate for information recording medium.